User blog:Samuli.seppanen/First velocity measurements on the cheiroballistra
Today I had my second round of shots using my cheiroballistra reconstruction after 3 years since the project was started. One of the reasons (besides being terribly busy with work and studies) for the slow pace of the project has been my attempt to make a scientific cheiroballistra reconstruction that actually works as a weapon, and to document the entire process very thoroughly. The slow pace has occasionally made it difficult to stay motivated, but finally the thing is shooting, and it's shooting reliably, so most of the hard work is finally behind. Nick's Orsova reconstruction has proven that torsion ballistas can have impressive performance, and I hope to show the real potential of the cheiroballistra as a weapon. Besides the obvious motivational effect of his reconstruction, Nick has been a big help when discussing all things related to ballistas. Here are some pictures from the first round of shots: Anyways, unlike the first round from a few weeks back this time I shot the bolts through a chronograph. To ensure that the ridicuously expensive chronograph lights were not in danger, I hacked together some shielding for them from L-shaped steel and birch plank. Today's results were pretty good, given that the machine is not tuned near it's maximum capacity. I shot 3 series of 5 shots with the same bolt that weighed exactly 30 grams: * Draw 21 cm below maximum: 44.44 m/s (29.63 J) * Draw 14 cm below maximum: 48.84 m/s (35.79 J) * Draw 7 cm below maximum: 53.75 m/s (43.34 J) The rapidly reducing string angle prevented a longer draw. I will have to make ~2 cm longer bowstring to be able to reach full draw. As comparison, the best results are almost identical to what I've managed to get from my 150 pound crossbow with ~27.5 cm draw using a same weight bolt. I did try to shoot a significantly lighter bolt (~12 gram) to see if velocity would go up significantly. However, it turned out that the butt of the bolt was too thin, and the second shot ended up being a dry fire: both of the arms fell off from the spring bundles and the bolt is yet to be found. Fortunately no ballista parts were damaged, and I was cautious enough to be hiding behind a wooden shield and pulling the trigger using a cord. In the end, the whole spectacle wasn't that dramatic, really. All in all, the cheiroballistra was a joy to use. Cocking it was straightforward with just stomach pressure, which was to be expected with these first fairly low-power shots. However, as power levels are ramped up, it may become necessary to grab ballista's arms and assist the slider pullback with both hands at the end of the draw. I actually tried this method today, and it made pullback a lot easier. What struck me most was how silent the ballista was compared to crossbows of equal power, which tend to have a very violent snap when shot. It seems that being able to lock the washers at 15 degree intervals is granular enough for the cheiroballistra: the reason is probably the slimness of the springs compared to, say, Nick's Orsova reconstruction. The natural way to shoot the cheiroballistra is to grab the thickest part of the case with one's left hand, support the left elbow to the hip and anchor the crescent-shaped piece behind the neck. This gives a perfect view of the target along the slider. Due to the elbow support the hefty weight of the ballista does not matter much. In fact, it helps keep the aim steady. The angle of the shot can be adjusted by bending slightly backwards. The balance point of the cheiroballistra is very near the little ladder, which also makes it easy to run around with the thing while holding it from the curve in the little arch. Now, quite a few things have to be tuned to produce better efficiency and more power: * More washer rotation * Elm arms with absolutely straight grain at the back and thinner hooks * Lighter and longer bowstring The wedges holding the little ladder, little arch and the field-frames together seem to become loose pretty quickly, so some sort of locking mechanism is needed. Or maybe I just have to beat them real tight. I eventually need to rebuild the field-frames, which have become ugly due to all the reinforcements I've had to add. Also, the curves in the curved field-frame bars are way too deep, which prevents using them to stop the arms at the end of the shot. In addition to that, there are a number of other fixes and enhancements that have to be made. In the end I may end up making all the parts again. Eventually I'll make sinew springs, but that'll have to wait until all the "bugs" in the ballista have been ironed out completely. If you're curious, I've calculated that it takes ~80 hours to make the sinew cord from deer leg tendons, and ~40 hours if using deer backstraps instead. I've tested the whole sinew cord making process from start to finish and it works, so it's just hard work from now on. Category:Blog posts Category:Backup Category:Statistics